If you have 14 days a good place to start would be Leon, a city with much to see so a days rest before you start. It also has good connections to Madrid. The first couple of days walking are not the most interesting from there but they will give you time for your body to adjust before you hit the hills. The journey from Astorga onwards covers some of the most picturesque parts of the camino.
Villafranca is a good town to linger in or maybe take a short days walk up the valley to make the climb to O Cebreiro a shorter day.
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You could start at Roncesvalles and do the first four or five days there - it is a very beautiful part of the Camino and a pity to miss it. Then bus it to Leon and do a few days from there. Leave about 5 days to do the last 120kms from Sarria so that you can earn your Compostela.
I have to agree with William: Leon is a good place to start for your average 'fit' walker.
You of course have to gear it to your personal abilities and the prevailing weather. Injuries incurred (blisters, twisted ankle) can put a real crimp in your walking style. See if you can build in a couple of buffer days to make sure you will make Santiago in your time allowed.
Astorga is a reasonable altenative if you wish to take a slightly more leisurely approach to the Road. Both train (RENFE) and bus lines service Astorga, so it is easy to get there.
Villafranca del Bierzo could be the next starting place for your Journey, but I don't see a train stop there. Ponferrada, however, is serviced by RENFE. Both have bus service in and out.
Some folks like to start in O Cebreiro, but it is not as easily accessible. There is bus service to Pedrafita do Cebreiro, an easy 4 km short of O Cebreiro.
Watever you do, whatever you decide, have a great Camino.